Share custom compositor nodes among blend files
You could customize your start up file. 
1. Open your start up file
2. Append the node groups you use a lot, click on the F in the node groups and delete the node groups.
3. Go to your default screen
4. CTRL + U save sart up file.
5. Close your file
6. Open a new file, go to the compositor, SHIFT + A --> group --> your node groups